1. Field of the Invention
The present invention relates to a display device, and more particularly to backlight brightness control for a display device, such as a liquid crystal display (LCD) device. This application claims the benefit of priority based on Japanese Patent Application No. 2006-335383, filed on Dec. 13, 2006, the disclosure of which is incorporated herein by reference.
2. Description of the Related Art
Liquid crystal display devices are often used in mobile information devices, such as cell phones, due to the reduced dimension. Recent requirements as to mobile information devices include not only providing function-limited substitutes for general information device, such as desktop computers, but also providing sufficient performances comparable to desktop systems.
For instance, one requirement as to the screen display of mobile information devices is to provide improved backlight brightness adjustment. Japanese Laid-Open Patent Application No. 2005-123097 discloses a backlight control technique for liquid crystal display devices.
FIG. 1 is a block diagram illustrating the configuration of a liquid crystal display device disclosed in this application. The disclosed liquid crystal display is provided with a liquid crystal display panel 41, a data line driver circuit 42, a scan line driver circuit 43, a controller 44, a lighting timing controller 45, a set of inverters 461 to 464, a set of frequency controllers 471 to 474, and a set of backlights 481 to 484 each incorporating a cold-cathode tube. Display pixels 50, each including a TFT (thin film transistor) 51 and a pixel electrode 52 opposed to a common electrode COM, are disposed on the liquid crystal display panel 41. The data line driver circuit 42 drives data lines X1 to Xm of the liquid crystal display panel 41, and the scan line driver circuit 43 drives scan lines Y1 to Ym of the liquid crystal display panel 41.
In order to stably and efficiently turn on the cold-cathode tube backlights 481 to 484, the lighting timing controller 45 and the frequency controllers 471 to 474 provide frequency control for the drive pulse voltages e1 to e4 fed from the inverters 461 to 464 to the backlights 481 to 484. In this liquid crystal display device, the frequencies of the drive pulse voltages e1 to e4 are increased at the initial stage of lighting the backlights 481 to 484, and then decreased after the operation of the backlights 481 to 484 is stabilized.
One known method for controlling the backlight brightness is PWM (pulse width modulation) control, which involves feeding a PWM-modulated drive signal to the backlight, wherein the PWM-modulated drive signal is an ON/OFF-controlled rectangular pulse signal with the pulse width controlled in accordance with the desired brightness. This method is often applied to the backlight brightness control for an LED backlight. The backlight is turned on when the PWM-modulated drive signal is pulled up to “H”, and the backlight is turned off when the PWM-modulated drive signal is pulled down to “L”. The brightness of the backlight is controlled by the duty ratio of the PWM-modulated drive signal.
Conventionally, the PWM control of the backlight brightness is clocked by a dedicated clock signal. This undesirably requires feeding to an LCD driver at least two clock signals: a clock signal dedicatedly used for the PWM control and another clock signal used for data transmission of pixel data, which are data indicating the grayscale levels of the respective image pixels of frame images to be displayed; the latter is often referred to as the “dot clock”. The use of two clock signals is undesirable for satisfying the requirement of the power consumption reduction; generating an increased number of clock signals undesirably increases the power consumption. The increased power consumption is one of the issues of the mobile information devices, from the background of the increased amount of data transmission for satisfying the high resolution requirement.